Implement ErrorEvent, Window/Worker.onerror, and proper JS error reporters

[jdm]

Background information: SpiderMonkey (the JavaScript engine) exposes a user-specifiable callback executing JS that throws an exception that is not caught. At the moment, the rust-mozjs library (which wraps the C-based mozjs aka SpiderMonkey) sets this callback to a simple function that logs the exception to the terminal. We want to replace this in Servo with a method that implements the error reporting specification for web browsers as described below.

Spec: http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#runtime-script-errors
ErrorEvent interface: http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#errorevent
OnError algorithm: http://www.whatwg.org/specs/web-apps/current-work/multipage/webappapis.html#the-event-handler-processing-algorithm

This will require fixing an XXXjdm comment in set_event_handler_uncompiled, too, and implementing a new SpiderMonkey error reporter that invokes the actual ErrorEvent dispatch to the window/worker.

[jdm]

This is potentially being worked on by a team of students, so anyone interested in working on it should contact me first.

[jdm]

Initial step: Build Servo, add a new ErrorEvent WebIDL file and corresponding Rust implementation (stubbing whatever methods are necessary), rebuild Rust. Use other event types, such as event.rs and uievent.rs as models.

• Create the ErrorEvent interface
• Replicate the existing logging error reporter in script code and use that instead
• Get the global object (JS_GetGlobalObject) in the error reporter and unwrap it (global_object_for_js_object) into a Window
• Create an ErrorEvent and dispatch it to the window (look for uses of dispatch_event_with_target for examples)
• Implement the remainder of the error event dispatching algorithm linked above
• Do all the same work for Workers, too (see workerglobalscope.rs)

[jwilm]

@jdm is this still being worked on by students? There's a bunch of easy wins in the web-platform-tests if we can finish this. I would be happy to complete this if it's not being worked on.

[jdm]

Nope, it's available!

[jwilm]

Sweet. I'll get on it tonight.

[nox]

Blocks #2185.

[nox]

@jdm I have some questions:

• Should EventTarget get a new flag for "in error reporting mode"?
• Should ErrorEvent get a new flag for "handled"/"not handled"?
• How and where should we handle "muted errors"?
• Does JSErrorReport::filename correspond to the absolute URL mentioned in step 5 of the spec?
• Do we actually need the "report the error" algorithm, or should we implement only the "report the exception" one used on the global object?